Boilers are widely used to generate steam for numerous applications. In the pulp and paper industries, recovery boilers are used to burn the liquor produced in a kraft pulp making process. Such boilers require combustion air. The current practice for introducing combustion air into the kraft recovery boilers involves injection of the air at two or more elevations in the furnace of the boiler. At the lowest elevation, air is injected through ports in all four walls. At higher elevations, air is injected through ports in all four walls or in two opposite walls of the furnace. The port openings which form the air jets are usually rectangular.
Conventional boiler systems have at least three basic deficiencies:
In some cases, the jet port openings are so small that when upflowing combustion gases in the furnace come from below the openings, an individual jet stream coming from a port does not have enough momentum to enable the jet stream to reach the centre of the furnace before the jet is deflected upwards.
The combustion air is usually injected in such a way that the jet streams of combustion air interfere with each other, and the interference causes upward deflection of the jet streams. Two locations where such interference can occur are at the centre of the furnace, where the jet streams from opposite walls of the furnace may meet head on, if they penetrate before being deflected upwards; and in the corners of the furnace, where the jet streams meet at right angles and interfere with one another.
When jet streams meet head on and are directed upwards, they tend to be repelled somewhat such that there is an isolated space between their paths, and hence there is restricted mixing in these spaces.
Due to the lack of momentum of the jet streams and because of the interference between the jet streams, as described, gas in the centre of the furnace is directed sharply upwards with somewhat of a diverging pattern. The result is a central updraft core of high gas velocity relative to the average upward gas velocity at any one horizontal cross-section of the furnace. This central updraft core begins at the primary air level. The updraft core has associated with it a recirculating downflow by the furnace walls, which adds to the upward velocity of the gas in the centre of the furnace. It has been found that air jets located more than one to two metres above the primary air level have great difficulty penetrating this central updraft core. The chemical composition in this updraft core is unsuitable for thorough combustion because it contains a high concentration of combustibles and little oxygen for combustion.
The primary jets, located at the lowest elevation in the furnace, are the main factor in initiating the recirculating pattern and the adverse central updraft. In essentially all current recovery boiler designs, the primary air is introduced more or less equally through multiple openings in all four walls thereby forming a plane jet stream off each wall. These four plane jet streams meet in the central region of the furnace and rise together. As the jet streams issue from the ports, they entrain surrounding gases. Since the upflow of volatiles from the char bed of the furnace is limited in volume, gases are necessarily drawn down the furnace walls in order to continually replace the gases that are entrained into the upwardly flowing jet streams. This action sets up a recirculating flow pattern in the furnace.
In boilers which have only one air entry level below the liquor spray level, such as older Combustion Engineering-type (CE-type) boilers, the central updraft core has been found to occupy approximately 1/9 of the horizontal cross-sectional area in the lower furnace. This core extends up through the elevation where the liquor spray is introduced. The top of the recirculating pattern occurs some height above the liquor spray level in the boiler, at an elevation corresponding approximately to the uppermost level of air injection in such boilers-designated the tertiary air level for the purposes of this discussion. The air jets introduced at this upper air level have been found to have little influence on the recirculating pattern.
Most boilers with two levels of air entry below the liquor sprayers, such as older Babcock & Wilcox-type (B&W-type) boilers and the newer CE-type boilers, have primary and secondary air ports on all four walls, with the air at a given air level being introduced more or less equally on each wall. The introduction of secondary air more or less equally from all four walls in such boilers reinforces tee detrimental central updraft core phenomenon.
One of the major operational problems in kraft recovery boilers is the formation of fireside deposits on the pendent heat transfer surfaces in the upper part of the boiler. The most troublesome deposits occur in the superheater and the first part of the boiler bank. These deposits are formed mainly by particles that originate from the gas entrainment of some of the liquor spray particles. The mass of a particle that can be entrained in a gas varies with about the sixth power of gas velocity. Therefore, from a conceptual perspective, it is important to minimize gas velocity extremes. As the liquor spray particles fall towards the bottom of the furnace, they swell and lose weight, becoming less dense, and therefore become easier to entrain. Therefore, the most sensitive area for entrainment is at the char bed/primary air entry level of the furnace. A second critical area is where there is a secondary level of air entry just above the char bed. Most of the particles that are entrained upwards into the region above the liquor spray level by the upwardly flowing gases are essentially destined to be carried out of the furnace by the furnace exit gas. Therefore, for the air introduced above the liquor spray level, gas velocity is not as much of a concern relative to minimizing fireside deposition.
Char bed control is a major operational concern with kraft recovery boilers. The char is formed as liquor spray particles burn in the furnace. The char is partially burned in flight, as it falls to the bottom of the furnace, but the last part of the carbon in the char is burned out on top of the char bed that covers the bottom of the furnace. One of the main functions of the primary air jets is to supply the oxygen to burn the char on the surface of the bed. The heterogeneous combustion of the char on the bed is limited by the mass transfer of oxygen, by diffusion. If the primary air jets are ineffective at supplying oxygen to the char, the bed grows in size. When this occurs, the boiler operator increases the temperature and/or pressure of the liquor in the spray guns, so that the liquor spray has smaller particles. With smaller particles more of the char is burned in flight so less has to be burned on the char bed. Increased liquor spray temperature, while it does control the char bed size, has the disadvantage that the smaller spray particles are easier to entrain by the upflowing furnace gases, resulting in an increase in the rate of formation of fireside deposits.
In one recovery boiler retrofit, designed by the inventors, some of the primary air ports were enlarged, to decrease the velocity of the primary air jets, with the intention of minimizing the scouring of char particles off the char bed by the primary air jets. Operating experience on the boiler after the retrofit indicated that low-velocity primary jets are somewhat impractical because they are not as effective at controlling char bed size and shape as high-velocity primary jets. The theory of low-velocity primary jets would be more practical if liquor spray guns produced particles essentially all one size. However, the guns produce a range of particle sizes, from fine to coarse. The large particles presumably cause the problems with control of char bed size. The fine particles are entrained by the furnace gases and are the source of the material that forms the fireside deposits on the heating surfaces in the upper part of the boiler. Both before and after the retrofit, a flat metal bar was inserted into the boiler, in the upper heating surfaces; on removing the bar after a short time, unburned black liquor spray particles were observed. Also, observations with the two char bed imaging cameras after the retrofit indicated that conditions in the lower furnace were quite quiet and there did not seem to be much carryover from the region just above the char bed. Nonetheless, the fireside deposits continued to form. These observations lead to the conclusion that the fine liquor spray particles were the dominant source of material for fireside deposits, rather than particles scoured off the char bed. Furthermore, the fine liquor spray particles seemed to be carried directly upwards. With the low-velocity primary jets, char bed control deteriorated, so higher liquor spray temperatures were required to compensate. Higher liquor temperature caused more fine spray particles and therefore more direct carryover from the liquor spray level. Subsequent to this, the enlarged primary ports were dampened off, thereby forcing the air to the remaining smaller primary ports. This increased the velocity of the air jets from the primary ports remaining open. With the increased velocity of the primary jets, char bed control improved, allowing the liquor temperature to be decreased. In retrospect, the enlarged primary ports were a conceptual error. Rather, it would have been better to use high-velocity primary jets to control the char bed size, thereby allowing a lower liquor temperature and pressure which would generate fewer finer particles, and in turn decrease carryover. Therefore, while low primary jet velocities are indicated theoretically, practical considerations of char bed control make high velocity primary jets more beneficial overall.
Secondary air is provided above the char bed. The function of these air jets is to provide oxygen for the combustion of the volatiles such as carbon monoxide and hydrogen gasified from the liquor spray particles. Here the main concern is to provide the necessary mixing of the combustible gases and the air, while minimizing gas velocity extremes that aggravate the entrainment of liquor spray particles. These jets are not intended to impinge on the char bed, so they do not have a direct char bed control function. Therefore, low velocity is indicated for the secondary air jets.
U.S. Pat. No. 2,416,462 Wilcoxson, discloses a concept involving an interlacing pattern of unopposed jets at the tertiary air level of a furnace (above the liquor sprayers) but it appears such interlacing was done without full appreciation as to the effects of interlacing. No interlacing at the primary and secondary levels of the boiler is disclosed.
Fridley and Barsin [Fridley, M. W. and Barsin, J. A., "Upgrading the Combustion System of a 1956 Vintage Recovery Steam Generator", Tappi Journal , Mar., 1988, page 63 and Fridley, M. W. and Barsin, J. A., "Upgrading a 1956 Vintage Recovery Steam Generator-II", Technical Section, Canadian Pulp and Paper Industry, 1988 Annual Meeting , Montreal] described modifications to an older CE-type boiler in 1986, to implement fully interlaced, unopposed, air jets at the secondary level, below the liquor spray level. There was an improvement in boiler performance. They claimed a decrease in liquor spray carryover. Recent B & W designs of recovery boiler air systems also incorporate this full interlacing of air jets at the secondary level. None of these designs extend interlacing to the primary level.